1. Field of the Invention
The invention pertains generally to the thermoremanent formation of a graphic image on a magnetizable surface and more particularly to a flash lamp configuration including means for uniformly irradiating the magnetizable surface.
2. Prior Art
Recently, there was developed an advantageous full frame thermoremanent transfer station for a magnetic imaging system. This transfer station, its utilization, and advantages are more fully described in the above referenced co-pending application, the disclosure of which is herein incorporated by reference.
The transfer station utilizes a transparent cylindrical carriage means to provide a full frame thermomagnetic transfer from a slave web to a master web. Coaxially located within the cylinder is an elongated flash lamp that produces the energy flash necessary for the transfer. Also provided are dual web transport rollers, each co-acting with a corresponding locking assembly.
Although, the transfer station configuration disclosed solves many of the problems found in the prior art of flash transferring an image including timing, registration, slippage, and spacing difficulties, it could be improved by providing a means to insure the uniformity of the energy profile from the flash lamp over the entire magnetic transfer surface.
Generally, this problem has not been addressed by the prior art because the thermomagnetic copying process has been thought of as threshold in nature. The theory was as long as all magnetic areas to be erased or written were heated beyond the Curie temperature there was no necessity to reduce the differential in energies one area might incur in relation to another.
However, it has been found that the uniformity of exposure in a thermomagnetic imaging system is important. If the exposure is uniform, the process becomes more efficient as the peak energy output from the flash lamp can be adjusted to heat the material just beyond the Curie temperature. Normally, the peak output must be somewhat higher to allow for the flash lamp envelope non-uniformity at the edges of a transfer document. Another advantage of uniformity is that if the lower peak power is used in imaging, the magnetic surface will cool faster.
Primarily, the need for uniformity of the energy profile is produced by the material constraints of the imaging system used. The masks used for thermomagnetic imaging are usually opaque in image areas and must absorb the energy radiations from the flash lamp to mask premagnetized surfaces. If the energy profile is relatively uniform across the recording surface, the masks may have a lower optical density than if differentials of high peak energies and illuminations had to be deflected. Also, the particular materials used in the imaging process are not so critical as much less heat need be absorbed by the opaque masking materials.